INFANCY AND PARENTAL CARE 



333 



of the species, whereas other kinds of doings would probably lead to 

 the extinction of the species. Some plants behaved in a certain way 

 in past ages, and their progeny to-day occupy the surface of the 

 earth. Other plants behaved quite otherwise, and we know of them 

 only by the traces they have left in the ancient rocks of the hills. 



Fig. 157. The four-spined stickleback (Apeltes quadracus) 



The adult fish swims about and through the nest, guarding the eggs while they 



are hatching 



387. Infancy among animals. When we study the pro- 

 longation of infancy among animals, we find that the advan- 

 tages of a protected and cherished youth are even more marked 

 there than they are among plants. 



Among most of the lower animals the mother lays large 

 numbers of eggs — in the water, on leaves, in the soil — and 

 abandons them. But toward the upper end of many series of 



